On the “NewsHour” tonight: Republicans stand back as their presidential nominee of four years ago levels a blistering attack on this year’s GOP front-runner, Donald Trump.

Also ahead this Thursday: fighting fire with fire. Researchers turn to mosquitoes as a weapon against Zika.

KARLA TEPEDINO, Oxitec: The beauty of this technique is that it can reach the mosquitoes where no other technique can find it. We are using mosquitoes to fight themselves.

JUDY WOODRUFF: Plus, women on Wall Street — the former managing director of a New York investment bank on the rampant sexism she and others endured.

MAUREEN SHERRY, Author, “Opening Belle”: If women were walking by in a skirt, he would throw himself on the floor, pretend to look up their skirt. And they would — he would get the laugh, you know, from the other guys.

JUDY WOODRUFF: All that and more on tonight’s “PBS NewsHour.”

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JUDY WOODRUFF: In the day’s other news, both Apple and the FBI picked up allies today in their legal fight over encryption. There was word that Google, Facebook, and Microsoft will oppose efforts to make Apple unlock an iPhone. It was used by Syed Farook in the mass shootings in San Bernardino, California.

At the same time, relatives of six of the victims filed a brief supporting the FBI’s position.

The state of Florida is ready to reinstate the death penalty. State lawmakers voted today to replace a law that the U.S. Supreme Court struck down in January. The old law required a simple majority of jurors to recommend a death sentence. The new one says at least 10 of the 12 jurors must agree.

From the United Nations today, hopeful words on Syria. A special envoy declared that a cease-fire is mostly holding after six days. In Geneva, Staffan de Mistura said the truce is fragile, but has — quote — “greatly reduced the violence.”

STAFFAN DE MISTURA, UN Special Envoy for Syria: This is good news for many Syrian people. Unfortunately, we have to admit, like in every cessation of hostilities or cease-fire, and in particular in this one, there are still a number of places where fighting has continued, including parts of Hama, Homs, Latakia, and Damascus.

JUDY WOODRUFF: At the same time, Amnesty International charged Russian and Syrian government forces are deliberately targeting hospitals in and around Aleppo. It said that amounts to war crimes.

North Korea has answered a new round of U.N. sanctions by firing half-a-dozen short-range projectiles into the sea. South Korea’s military says they were fired today near the eastern town of Wonsan and flew less than 100 miles. Later, the North announced that its leader, Kim Jong-un, has ordered the military to be ready to fire nuclear weapons at any time.

European leaders sounded increasingly desperate today to deal with the mounting migrant crisis in Greece. Nearly 32,000 people are stranded, and largely blocked from moving deeper into the continent.

James Mates of Independent Television News is there on the ground.

JAMES MATES: Don’t come to Greece, they have been told. Don’t believe the smugglers. You won’t get any further. But, still, they come, 600 more pouring onto the keys in the Port of Piraeus this morning.

But for these new arrivals, things have changed. No longer are they being waved through on their journey north. For many now, a tent on the dockside is the end of the road. The main ferry terminal has been home to several hundred for more than a week now, many young children, for whom the longed-for passage to Germany may never happen.

This is what happens when you try and block a human tide as it flows northwards. Just a week or so ago, all of these people would have moved on across the Macedonian border and towards Northern Europe. Now, well, they’re struck here.

KATERINA KIDDI, UN Refugee Agency: It’s obvious that Greece cannot do it alone. It was never a Greek problem. It was always a European and a global problem. You cannot expect from one country to share this big responsibility.

JAMES MATES: Senior E.U. officials are in Athens today trying to reassure the Greeks they’re not alone. But the words are aimed at those still thinking of traveling.

DONALD TUSK, President, European Council: Do not come to Europe. Do not believe the smugglers. Do not risk your lives and your money. It is all for nothing.

JAMES MATES: But for those already here, Greece may no longer be a transit country. In Athens’ picturesque squares, many hundreds now sleep in the open. They can’t move on. They won’t go back.

But like Zohreg Gasam and her four sisters from Afghanistan, they have found no welcome.

ZOHREG GASAM, Refugee: We are tonight sleeping there, and we not have two…

JAMES MATES: Your blankets.

ZOHREG GASAM: Yes, blankets, yes. The winter was really cool.

JAMES MATES: There was once a warm welcome for refugees when they came off the boats here, but that was when they were able to move on through. Now the burden seems to be falling on Greeks alone. And patience and tolerance are becoming harder to find.

JUDY WOODRUFF: The migrant crisis will be the main subject at a European Union summit on Monday.

Back in this country, Oregon is set to become the first state to eliminate coal from its energy supply by law. The state legislature gave final approval yesterday to the move. It takes full effect by 2030. Governor Kate Brown has indicated she will sign the bill.

Wall Street made some modest progress today. The Dow Jones industrial average gained 44 points to close near 16944. The Nasdaq rose four. And the S&P 500 added nearly seven.

And President Obama says he will do something few presidents have done: stay in Washington after he leaves office. He said today in Milwaukee that he wants Sasha, his younger daughter, to finish high school before the family moves. She’s now in the ninth grade.

Still to come on the “NewsHour”: two leading Republican voices on Mitt Romney and the feverish efforts to stop Donald Trump; mosquitoes that fight the virus they spread; the horrifying ways ISIS is using child soldiers; and much more.

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